The invention relates to coherent demodulation of signals which have been modulated using offset quaternary phase shift keying (OQPSK), which class of keying includes the important case of minimum shift keying (MSK).
This type of keying is being used more and more for microwave beam transmission since it offers important advantages, including the advantage of presenting a constant envelope.
The circuit of the invention makes it possible to obtain the phase synchronization required for coherent demodulation in a manner which is effective and simple. The quality of such a circuit has an important influence on the performance of a receiver in terms of error rate.
Phase synchronization for OQPSK type systems gives rise to problems that do not exist in other comparable keying systems (e.g. QPSK and BPSK=Binary Phase Shift Keying).
There are several types of circuit for achieving phase synchronization with this OQPSK class of keying.
The most closely-related types are referred to as follows:
DD: "Decision Directed" PA1 NDD: "Non-decision Directed" PA1 MDD: "Modified Decision Directed". PA1 a four-phase demodulator; PA1 a processing module; and PA1 a phase error calculating circuit followed by a phase correcting circuit which delivers a phase error correction signal.
In which decisions concerning the symbols are used when calculating the error signal for servo-controlling the loop; and PA2 In which such decisions are not used.
An NDD circuit provides performance which is less good than a DD circuit (using the criterion of residual variance in phase error).
The circuit of the invention makes use of decisions and is therefore designated below as:
In DD and in NDD circuits, there remains a degree of residual phase error noise of non-thermal origin which cannot be reduced for given loop bandwidth. In other words, when the signal-to-noise ratio tends towards infinity, there remains non-zero fluctuation in the phase error which gives rise to a residual error rate.
This residual noise can be reduced to an arbitrary extent by reducing the loop bandwidth, but that prevents the system from tracking random keying which is mostly due to the local oscillators at the transmission end and at the reception end, and which justifies the use of a wide loop bandwidth which may optionally be adaptive as a function of the signal/noise ratio.
The invention serves to reduce this residual noise very considerably and to eliminate it completely in the special case of MSK.